This invention relates generally to production of metals by reduction of ores containing their oxides. More particularly, the invention relates to a smelting reduction method in which metal oxide ore, such as iron ore, is subjected in a solid state to a preliminary reduction (hereinafter referred to as prereduction) step in a prereduction furnace and thereafter melted in a smelting reduction furnace thereby to carry out final reduction of the ore. The invention concerns a smelting reduction method by which, particularly, the rate of energy utilization is increased, and the consumption of the reaction materials such as coal, oxygen, and lime is greatly reduced.
In the smelting reduction method, in general, metal oxide ore, such as iron ore (iron oxide), is reduced in a molten state thereby to produce iron or ferroalloy. Because of the promising possibility of its adaptation to coping with the further situations of raw materials and energy, this area of technology has recently attracted much attention, and research and development for its reduction to practice is being carried forward.
The principal advantageous features which this method affords as an iron producing method, in comparison with the blast furnace process, are use of low-price raw materials, reduction of preparatory processing steps such as sintering or pelletizing particulate ore, and miniaturization of necessary equipment. In addition, as a method in the production of ferroalloys, it has almost no dependency on the use of electric energy.
While various processes for practicing this smelting reduction method have been proposed, and the reduction furnaces used therein are of diverse form, the smelting reduction furnace of the metal smelting type is a representative form. In the case of a reducing furnace of this type for producing iron, for example, iron ore, together with coal and oxygen, is charged into molten iron bath, and the ore is thus reduced to obtain molten iron (pig iron). However, the reaction is rapid, it being possible to accomplish reduction at a rate which is 100 times or more rapid than in reduction of the ore in solid state, and the required equipment is of simple type. For these and other reasons, furnaces of this type are widely used in many processes.
On its debit side, a smelting reduction furnace of this type has the disadvantage of an extremely poor rate of utilization of energy. The fundamental reaction formula representing the reduction of iron oxides in a furnace of this type is as follows. ##STR1## Since the applied energy in this formula is the heat quantity of combustion of C (carbon), when it is calculated from the quantity of generated heat of C (8,100 kcal/kg), its value becomes 1.293.times.8,100=10,470 kcal. On the other hand, the heat quantity which has been effectively utilized is the sum of 1,759 kcal, the quantity of heat for reduction of Fe.sub.2 O.sub.3 (1 kg.), and 239 kcal, the heat quantity for melting Fe, that is, the total value 1,998 kcal.
Therefore, the rate of utilization of the energy applied is 1,998/10,470, that is, only 19 percent. Almost all of the remainder is discharged as exhaust gas. Accordingly, in order to increase the rate of utilization of energy, it is necessary to utilize the energy held by this exhaust gas.
A possible measure for this purpose is the so-called secondary combustion technique in which oxygen (or gas containing oxygen) is blown into the gas space part within the smelting reduction furnace thereby to cause combustion of a portion of the combustible gas issuing from the molten metal surface, and one portion of the heat thus generated is recovered and returned into the molten metal, whereby the energy utilization rate of the reduction furnace is increased. This measure utilizes the fact that, the combustion heat generated in the conversion of CO into CO.sub.2 is 2.5 times the combustion heat generated during the conversion of C into CO.
In the case where the secondary combustion rate is 30%, that is, when 30 % of the CO gas emitted from the melt within the furnace is caused to undergo combustion and thus be converted into CO.sub.2, and the temperature of the gas within the furnace is set at 1,600.degree. C., the fundamental formula of the reaction within the furnace becomes as follows. ##STR2##
In this case, since the added energy is 0.679.times.8,100=5,500 kcal, the energy utilization rate becomes 36%. While this is a great improvement over the rate obtainable in the case where secondary combustion is not carried out, it is still insufficient. Elevating the secondary combustion rate to an extreme degree gives rise to an excessive rise in the temperature within the smelting reduction furnace and causes a problem in that the serviceable life of the refractories is shortened. Therefore, in order to further increase the energy utilization rate, the introduction of a newer technology is necessary.
As a consequence, a method wherein the raw-material ore is subjected to preparatory reduction or prereduction has been proposed. As mentioned hereinbefore, this method comprises prereducing the ore in its solid state in a prereduction furnace and then subjecting the ore to final reduction in a smelting reduction furnace as described above. For the reducing gas used in the prereduction furnace, high-temperature gas given off during the final reduction in the smelting reduction furnace is mainly used. For the prereduction furnace, a furnace of the fluidized bed type, in which the ore forms a fluidized bed and thus is contacted by and reacts with the above mentioned gas, is used in many cases. In this furnace, the reaction temperature is set at approximately 800.degree. C. so as to obtain a high reduction efficiency without causing sintering of the ore.
In a smelting reduction method of this character as practiced heretofore, in order to obtain as high reduction rate (prereduction rate) in the prereduction furnace as possible, efforts are being devoted toward development toward this goal. Ordinarily, the prereduction rate has been set at 70% or higher value. The term "reduction rate" as used herein designates the rate of decrease of oxygen on the basis of the metal oxide contained in the raw-material ore as reference. For example, in the case where Fe.sub.2 O.sub.3 is taken as reference (reduction rate 0%), the ore is reduced to Fe.sub.3 O.sub.4 at a reduction rate of 11.1%, to FeO at a rate of 33.3%, and to Fe at a rate of 100%.
The energy utilization rate in a process carried out in apparatus comprising a prereduction furnace and a smelting reduction furnace of this character will now be considered.
The fundamental formula representing the reduction reaction of iron oxide in the prereduction furnace is as follows. ##STR3## However, in order to reduce Fe.sub.2 O.sub.3 at 800.degree. C. to Fe, the CO/(CO+CO.sub.2) ratio in the gas at the outlet of the prereduction furnace must be maintained at 65% or higher value in accordance with the known Fe-CO equilibrium diagram (shown in FIG. 4 of the accompanying drawings briefly described hereinafter).
Accordingly, in order to increase the quantity of CO fed into the prereduction furnace in the case where the prereduction rate is to be 100% with this process, excess quantities of C and O.sub.2 must be added into the smelting reduction furnace. In this case, since the reaction within the smelting reduction furnace is an exothermic reaction, it is necessary to add a coolant into the furnace in order to maintain thermal equilibrium. For example, when the case where CO.sub.2 is used as the coolant is considered, the fundamental formulas therefor become as follows. ##STR4## In this case, since the energy added is the combustion heat possessed by C, that is, 0.768.times.8,100=6,221 kcal, the effective utilization rate of heat is 32%.
In the case where the prereduction rate is 75%, that is, where Fe.sub.2 O.sub.3 is reduced to FeO and Fe in the prereduction furnace, the formulas become as follows. ##STR5## The energy utilization rate is 39%.
In a process employing a prereduction furnace also, the secondary combustion technique is applied in some cases in the smelting reduction furnace as described above. However, since the prereduction rate is of a high value of 70% or more, it is necessary to hold the secondary combustion rate at 30% or less in order to secure the CO quantity in the gas for prereduction.
Thus, in a process employing a prereduction furnace and a smelting reduction furnace, the potential heat and the reductive capacity of the gas given off from the smelting reduction furnace are utilized in the prereduction furnace, and at the same time the sensible heat of the ore prereduced in the prereduction furnace is utilized in the smelting reduction furnace, that is, in the process itself, a portion of the energy is being recycled. In contrast, in the smelting reduction method of the prior art, the surplus energy not utilized in the process has been wasted in the exhaust gas.
The above consideration may be summarized as follows. In the known smelting reduction method employing a prereduction furnace and a smelting reduction furnace, the following characteristic features from the viewpoint of energy utilization were afforded.
(i) A prereduction rate of 70% or more.
(ii) A large quantity of surplus energy not utilized in the process has been wasted in the exhaust gas.
A serious problem accompanying the above described known smelting reduction method is that the rate of consumption of carbon (C) necessary for obtaining metal by reducing the metal ore (metal oxide) is high, that is, the energy utilization rate is low. For example, this value is low even in comparison with that of reduction of iron ore by the blast furnace method. For this reason, it is said that, with respect to the smelting reduction method, extensive commercialization thereof is difficult as long as this problem is not solved.
Because of the large consumption of carbon, the consumption of oxygen becomes large. Therefore, in actual practice, not only do adverse effects on production quantities such as the quantity of slag produced, the consumption of coal, and the loss of extracted metal into the slag arise, but the cost of equipment to cope with these effects also increases.
The energy utilization rates examined above are all based on the fundamental reaction formulas, that is, they are energy utilization rates under ideal conditions. In an actual reduction process, however, C is not pure carbon but is in the form of coal, and Fe.sub.2 O.sub.3 is also an iron ore containing impurities. Moreover, occurrences such as discharge of heat from the furnace structure (heat transmission loss) affect the results, whereby the actual rates become somewhat lower than these ideal rates.
Furthermore, since the prereduction rate is high in the conventional smelting reduction method, a prereduction furnace of large capacity is necessary. Another problem is that since metal iron is formed in the ore (prereduced iron), which tend to adhere to each other, the ore is formed into large lumps, whereby difficulties such as obstruction of reaction and transfer are encountered.